The present invention relates to a structure and method for strongly damping unwanted or damaging transverse fields in certain higher longitudal electromagnetic modes or frequencies in a radio frequency accelerating cavity.
Such cavities have a number of well-known applications including their use as accelerating devices in particle beam accelerators. As is well known, such accelerators are typically arranged with successive cavities in a linear configuration, or with the successive cavities arranged in a circular configuration. A major objective in designing and building modern particle beam accelerators of either the circular or linear type is to produce a beam of very high luminosity or brightness. The accelerated beam luminosity in a linear collider type of accelerator is known to be severely limited by the total beam power and thus is limited by the efficiency by which radio frequency power is transferred to the beam. Of course, the many other applications of RF accelerating cavities would also benefit from improvements in their RF power transferring efficiency. A primary restricting factor on that efficiency comes from the fields, known as wakefields, that are generated by the particle bunches in an accelerated beam being passed through the series of cavities in a particle beam accelerator structure, such as a linear accelerator or so called linac. Such wakefields act back on the bunches of particles and cause unacceptable momentum spread, distortion or deflection of the bunches.
In a linac that is loaded with many bunches of particles, each bunch is exposed to wakefields that are generated by itself and by a sum of earlier bunches of particles in the beam being accelerated through the cavity. The effects from the fields generated by a given bunch upon itself, at least for shorter particle bunches, are largely determined by the diameter of the beam apertures through the end walls of the accelerating cavity and through any iris structures positioned along the length of the cavity. Besides utilizing the largest possible diameters for such iris(es) and end wall apertures, there is not too much a cavity designer can do about the undesirable effects of the fields themselves, although BNS damping can be used to reduce those undesirable effects. The maximum fraction of RF energy extracted by one bunch of particles is then bounded to a few percent, the exact amount being dependent upon detailed design parameter assumptions. If more efficiency is required in the RF power transfer to the beam, it is necessary to use multiple bunches of particles.
Because the fields that are encountered by a given particle bunch passing through a beamline accelerating cavity are the sum of the fields from the preceeding particle bunches, it is desirable to strongly damp all unwanted and damaging electromagnetic modes in the cavities. The wakefields produced by the plurality of bunches of particles have many different frequencies that are not in general multiples of the bunch spacing and they will not add coherently. Thus for a large number of very small bunches of particles, the contributions of the wakefields are relatively less and higher effiencies of RF power transfer to the beam can be obtained.
Unfortunately in the design of cavities for use in particle beam collider applications, it is necessary to provide both relatively high individual particle bunch loading as well as to provide multiple bunches of particles, in order to obtained the needed high efficiency. In such cases, both momentum spread of the bunches and beam breakup become unacceptable unless the design of conventional accelerating cavities is significantly changed to overcome such problems. The present invention provides an accelerator cavity structure that strongly damps the wakefields generated by each bunch of accelerated particle bunches in a beam passing through the cavity, before succeeding bunches in the beam are introduced into the cavity. Such damping must be accomplished very rapidly; i.e., very low Q's are required to substantially eliminate all unwanted wakefields.